As wireless communication becomes increasingly common, more new frequency ranges are needed for different wireless systems. In a particular area, several of the following systems and frequency ranges may be operated simultaneously: EGSM 900 (880 to 960 MHz), GSM 1800 (1710 to 1880 MHz), GSM 1900 (1850 to 1990 MHz), WCDMA 2000 (1920 to 2170 MHz), US-GSM 850 (824 to 894 MHz), US-WCDMA 1900 (1850 to 1990 MHz) and US-WCDMA 1700/2100 (Tx 1710 to 1770 MHz, Rx 2110 to 2170 MHz). Moreover, different wireless local area network systems, like IEEE 802.11 and 802.16, and the wireless Bluetooth system are operating within the so-called unlicensed frequency range of 2400 to 2483.5 MHz. Furthermore, GPS satellite positioning system, having the frequency range of 1227/1575 MHz, is operating on a frequency range close to those used in other wireless systems.
In wireless transmission, both intra-system signals and intersystem signals, as well as noise, may interfere with the reception. Consequently, the quality of the received signals may be affected by quick interference due to various factors, such as multipath propagation, fading of transmitted signals, shadowing, near-far effect and co-channel interference. The interfering signals disturb the signal-processing methods used in telecommunications systems, due to which estimating the power or the bandwidth of a received signal, for instance, may be unsuccessful.
As different wireless systems, particularly those operating within the unlicensed frequency range, are becoming more commonly used, the receiver control necessitates, not only detection of interfering signals, but also estimation of the bandwidth, as well as the frequencies, of unknown narrowband signals for different purposes. When a signal is considered interference, the knowledge of that may be used to aid the interference excision, e.g., the transform selective interference suppression algorithm (TSISA). It may also be desirable to detect if a narrowband signal is present within a wideband signal, e.g., in systems with overlapping frequency ranges.
A commonly known method of frequency estimation of unknown narrowband signals, typically used in radar systems, is based on thresholding the radar signals with hysteresis. A drawback of the method is that the threshold setting is based on the noise present in the system. However, the estimation of noise increases the complexity of the algorithm and the estimation of noise may even be an impossible task in severe interference environment.
The forward consecutive mean excision (FCME) algorithm has been proposed earlier as an interference excision algorithm. Filtering the disturbing interference is often a prerequisite for the receivers to be able to operate at a sufficient accuracy. For instance in FFT (Fast Fourier Transformation) band-stop filters used in receivers it is important to find the suitable threshold value level, because the performance of the band-stop filter depends to a great extent on the correct threshold level setting.
The FCME algorithm is “blind” in the sense that it does not need to know the noise level in advance. The efficient FCME algorithm sets an excision threshold iteratively based on a threshold parameter. The threshold given by the FCME algorithm separates the set of samples into two sets. The samples below the threshold are caused by the noise and possibly by a spread spectrum signal. The samples above the threshold are caused by different interference impulses and signals and unknown narrowband signals. Some embodiments of the FCME algorithm are given in patent application PCT/F103/00536.
Although the FCME algorithm can efficiently detect interfering signal samples above the given threshold, it can locate narrowband signals only very approximately. Frequency components caused by the noise can exceed the threshold and cause false signals to be detected. In the noisy data, the signal can also be broken due to the possible destructive addition of the noise and the signal. The result is that the definition of the bandwidth and the frequencies of the signal may fail.